1. Field of the Invention
The present invention relates to beam-index type cathode ray tube devices wherein a phosphor screen is provided with a plurality of color phosphor stripes accompanied by index phosphor stripes on the inner surface of a screen panel portion which faces a front panel portion. Fluorescence from the index phosphor stripes is detected at the outside of the screen panel portion.
2. Description of the Prior Art
The prior art includes disclosures of the so-called reflex color cathode ray tube of the beam-index type as one type of color cathode ray tube device which can be adapted generally to form color television receivers of relatively small size. A reflex color cathode ray tube of the beam-index type has a front panel portion which allows colored light to pass therethrough and a screen panel portion which faces the front panel portion and is provided on its inner surface with a phosphor screen including a plurality of color phosphor stripes of the three primary colors and a plurality of index phosphor stripes. Color images displayed on the phosphor screen are observed through the front panel portion and fluorescence from each of the index phosphor stripes referred to a index fluorescence is received by a photosensitive portion provided at the outside of the screen portion panel with relation to the phosphor screen.
FIG. 1 shows a portion of a previously proposed reflex color cathode ray tube of the prior art. In this device, a phosphor screen is provided on the inner surface of a screen panel portion 13 which is positioned to face a front panel portion 11. The phosphor screen is formed with a plurality of index phosphor stripes 15 which are secured on the inner surface of the screen panel portion 13 and extend parallel to one another at regularly spaced intervals for emitting ultraviolet rays as index fluorescence in response to impingement of an electron beam thereon. A plurality of carbon stripes 17 is each affixed to the inner surface of the screen panel portion 13 and extend between each adjoining pair of index phosphor stripes 15, i.e., there are two phosphor stripes between two adjoining carbon stripes. A plurality of color phosphor stripes of the three primary colors including red phosphor stripes 19R which produce red fluorescence upon excitation by an electron beam, blue phosphor stripes 19B which produce blue fluorescence by excitation of the electron beam, and green phosphor stripes 19G which produce green fluorescence in response to excitation by the electron beam. These phosphor stripes are fixed on the carbon stripes 17 and extend parallel to each other alongside the index phosphor stripes 15 at regularly spaced intervals.
The tube also includes a luminescence receiving plate member 21 which faces the outer surface of the screen panel portion 13 for receiving the index fluorescence emitted by each of the index phosphor stripes 15. This luminescence-receiving plate member 21 is operative to emit secondary index fluorescence which may be detected by an index fluorescence detector such as a photodiode in response to the index fluorescence emitted from the index phosphor stripes 15 and direct the same to an index fluorescence detector. Accordingly, the index fluorescence emitted from each of the index phosphor stripes 15 is converted in wavelength by the luminescence receiving plate member 21 so that it may be detected by the index fluorescence detector. The luminescence receiving plate member 21 may be formed of an acrylic resin in which a specific phosphor, selected for its ability to absorb the index fluorescence from the index phosphor stripe 15 and emit the secondary index fluorescence, is dispersed.
In such a beam-index type color cathode ray tube, when the phosphor screen on the inner surface of the screen panel portion 13 is scanned by an electron beam emitted by an electron gun assembly, color images are displayed on the phosphor screen with the red, blue and green fluorescences emitted by the red, blue and green phosphor stripes 19R, 19B and 19G, respectively, and are observed through the front panel portion 11. At the same time, the index fluorescence emitted from each of the index phosphor stripes 15 impinges on the luminescence receiving plate member 21 through the screen panel portion 13 and the secondary index fluorescence produced in response to the fluorescence from the index phosphor stripe 15 is directed to the index fluorescence detector from the luminescence receiving plate member 21. An index signal used for modulating the phosphor screen with a color video signal is thus supplied to the electron gun assembly in response to momentary scanning positions of the electron beam on the phosphor screen.
In the previously described reflex color cathode ray tube of the beam-index type as described above, portions of the red, blue and green fluorescences emitted from the red, blue and green phosphor stripes 19R, 19B and 19G toward the screen panel portion 13 are extinguished by the carbon stripes 17 to prevent them from entering into the luminescence receiving plate member 21 through the screen panel portion 13 and thus causing the luminescence receiving plate member 21 to produce spurious secondary index fluorescence. Moreover, the index fluorescence emitted from the index phosphor stripe 15 is directed not only toward the screen panel portion 13 but also toward the front panel portion 11 and a portion of the index fluorescence emitted from the index phosphor stripe 15 toward the front panel portion 11 does not arrive at the luminescence receiving plate member 21 and therefore does not contribute to the production of the secondary index fluorescence which is detected by the index fluorescence detector. This means that the index fluorescence emitted from the index phosphor stripe 15 is reduced almost by half in the production of secondary index fluorescence. Accordingly, it is possible that the index fluorescence entering into the luminescence receiving plate member 21 from the index phosphor stripe 15 becomes insufficient in intensity to cause the luminescence receiving plate member 21 to produce the required secondary index fluorescence, and as a result, the secondary index fluorescence detected by the index fluorescence detector is insufficient in intensity. Consequently, an index signal with a superior signal-to-noise ratio cannot be obtained from the index fluorescence detector.
Furthermore, the part of the index fluorescence emitted from the index phosphor stripes 15 toward the front panel portion 11 is within the sight of observers together with the red, blue and green fluorescences emitted from the red, blue and green phosphor stripes 19R, 19B and 19G toward the front panel portion 11. This results in the disadvantage that the color purity of each of the red, blue and green fluorescences observed by the observers is deteriorated due to visible rays contained in the index fluorescence emitted from the index phosphor stripes 15. Particularly in the case where the electron beam for exciting each of the index phosphor stripes 15 is increased in density for the purpose of increasing the intensity of the index fluorescence entering into the luminescence receiving plate member 21, the portion of the index fluorescence emitted from the index phosphor stripe 15 toward the front panel portion 11 to be in the sight of observers together with the red, blue and green fluorescences is also increased in intensity. Accordingly, the deterioration in color purity of each of the red, blue and green fluorescences emitted from the phosphor stripes is objectionable. In addition, in this case, since the dark electron beam current in the cathode ray tube is increased, the black level of the color image on the phosphor screen is increased so as to deteriorate the quality of the color image.
For the purpose of avoiding the above-mentioned problems or disadvantages inherent in the reflex color cathode ray tube of the prior art, it has been suggested that such a reflex color cathode ray tube as shown in FIG. 1 be provided with a metallic reflecting layer to cover each of the index phosphor stripes fixed on the inner surface of the screen panel portion and each of the partial areas of the inner surface of the screen panel portion at which the index phosphor stripes are not attached. Then, a plurality of color phosphor stripes of the three primary colors is provided on the metallic reflecting layer so as to form a phosphor screen. In a reflex color cathode ray tube of this type which is provided with a metallic reflecting layer, the index fluorescence from each of the index phosphor stripes is reflected at the surface of the metallic reflecting layer on the side of the screen panel portion and is efficiently introduced into a luminescence receiving plate member disposed in facing relation to the outer surface of the screen panel portion without being directed toward the front panel portion. The red, blue and green fluorescences emitted from the color phosphor stripes are then reflected at the surface of the metallic reflecting layer on the side of the front panel portion and are directed efficiently toward the front panel portion. Consequently, the index fluorescence entering into the luminescence receiving plate member from each of the index phosphor stripes becomes sufficient in intensity to cause the luminescence receiving plate member to produce adequate secondary index fluorescence, and each of the red, blue and green fluorescences emitted from the color phosphor stripes is prevented from being deteriorated in color purity to the observer. The luminous efficiency of each of the color phosphor stripes is thus substantially improved.
However, in the case of the metallic reflecting layer being provided over the inner surface of the screen panel portion to cover the index phosphor stripes, there is a new problem presented as follows.
The metallic reflecting layer covering the index phosphor stripes is usually formed by means of vacuum evaporation on an intermediate thin layer of resin which is provided over the inner surface of the screen panel to cover the index phosphor stripes in order to fill up minute spaces among phosphor particles or the like at the surface of each of the index phosphor stripes and thereby form a smooth plane on each of the index phosphor stripes. Then, an intermediate thin layer of a resin is evaporated and dispelled in a heating process wherein the screen panel portion on which the phosphor screen has been formed is subject to thermal treatment.
When the intermediate thin layer of resin is evaporated and dispelled, the vapor resulting from the evaporation of the intermediate thin layer of plastic resin provided on each of the partial areas of the inner surface of the screen panel portion at which the index phosphor stripes are not fixed is prevented from escape by the metallic reflecting layer and as a result causes the metallic reflecting layer protruding from the inner surface of the screen panel portion to have blisters. The metallic reflecting layer provided with the blisters therein constitutes a defect in that a smooth surface of the metallic reflecting layer cannot be obtained and a further disadvantage that breaches or exfoliations of themetallic reflecting layer are likely to be caused at the blisters.